L-Ascorbic acid, or Vitamin C, is a water-soluble vitamin widely distributed in the plant and animal kingdom. It can be extracted from plant sources, such as paprika, Gladiolus leaves, rose hips, persimmon, and citrus fruit, or synthesized from L-xylose, L-galactose, or D-glucose. F. A. Loewus, L-Ascorbic Acid: Metabolism, Biosynthesis, Function, in The Biochemistry of Plants, Vol. 3, Academic Press, New York, 1980, pp. 77-99, reviews the biosynthesis and sources of L-ascorbic acid.
Although most species of animals synthesize L-ascorbic acid, humans and other primates, guinea pigs, fruit eating bats, some birds, and fish such as Coho salmon, rainbow trout channel catfish, and carp cannot. These animals require a dietary source of L-ascorbic acid to prevent scurvy. L-ascorbic acid deficiency in fish also causes scoliosis, lordosis, reduced weight gain, increased susceptibility to bacterial infection, dark skin color, fin erosion, and reduced formation of bone cartilage.
In the wild, fish obtain an adequate amount of L-ascorbic acid from aquatic organisms, such as algae. However, fish grown commercially in high density pens or ponds require supplementary L-ascorbic acid to prevent the problems caused by Vitamin C deficiency. L-Ascorbic acid is formulated into fish food before it is pelletized or extruded, but much of it is lost during production of the feed due to the high levels of moisture, heat (135.degree.-175.degree. C.), and pressure used in extrusion process. Even when ethylcellulose-coated L-ascorbic acid is used, about 50% of the L-ascorbic acid is lost. See R. T. Lovell, Trans. Am. Fish Soc., 107, 321-325 (1978).
Various algae, including Chlorella species, produce L-ascorbic acid. Aaronson, Arch. Microbiol. 112, 57-59 (1977) discloses light-grown Chlorella that contain up to 15 .mu.g L-ascorbic acid/mg of dry weight of cells, i.e., 1.5% by weight. Renstrom, Plant Sci. Letters, 28, 299-305 (1982/1983) discloses that L-ascorbic acid content in Chlorella is reported to be 6-82 .mu.mol/g dry weight for cells grown in light, i.e., 0.11 to 1.44% by weight. Chlorella pyrenoidosa Chick, culture (UTEX) No. 343, dark-grown for 0.5-6 days in a glucose-enriched medium produced about 3 .mu.mol of L-ascorbic acid per g dry weight (0.06 wt %) and four-fold this amount (0.24 wt %) in 12 hours when light was supplied. However, a need exists for biomass containing higher levels of L-ascorbic acid for use as a dietary supplement.